The present invention relates generally to devices and methods for measuring gas flow parameter(s) and specifically for measuring air flow parameter(s) for diagnosing lung performance.
Mechanical and electronic peak flow meters are used to monitor lung performance of patients having respiratory ailments, such as asthma. Mechanical peak flow meters in particular are used by patients to self-monitor lung performance. In such applications, lung performance parameters, such as peak air flow, are periodically recorded in a diary. If lung performance falls below a certain level or if the diary shows a deterioration in lung performance, the patient seeks medical assistance.
Mechanical peak flow meters are typically a spring-loaded device having a peak flow pointer attached to a plate-like spring that is displaced or slid laterally by the exhaled air of the patient to indicate the peak flow rate. Although such devices are inexpensive, the devices provide only a Peak Expiratory Flow Rate (PEFR) measurement and are unable to measure other desirable and important parameters, such as Forced Expiratory Volume in one second (FEV1) (the total volume of air exhaled by the patient over a one second interval) or Forced Expiratory Volume in 6 seconds (FEV6) (the total volume of air exhaled by the patient over a six second interval). The accuracy of such devices is often poor and typically deteriorates over time due. Mechanical devices are also unable to track peak flow measurements by writing the measurements to an electronic memory. The measurements must be manually recorded in the diary.
Electronic devices typically have a fixed or variable orifice and a pressure transducer located on one or both sides of the orifice. A fixed orifice is a reduced diameter passage (which can have any shape) having a fixed cross-sectional area that is independent of air flow rate. A variable orifice is a reduced diameter passage (which can have any shape) having a cross-sectional area that is dependent on air flow rate (e.g., the cross-sectional area increases as the flow rate increases). In either case, the orifice typically causes a back pressure to form in front of the orifice in response to air flow through the orifice. By measuring the back pressure and determining the pressure downstream of the orifice (which is typically ambient pressure), the pressure differential across the orifice can be obtained. The pressure differential permits not only the peak flow to be determined but also FEV1 and FEV6(when the pressure differential is measured as a function of time). Due to the use of expensive pressure transducers and associated electronics, electronic devices are typically too costly for individual patients in self-monitoring applications. As a result, patients are unable to monitor important lung performance parameters, such as FEV1 or FEV6.
These and other needs are addressed by the devices and methods of the present invention. The present invention is directed generally to an inexpensive device and method for measuring an (expiratory) air flow parameter, such as PEFR, FEV1, FEV6, Forced Vital Capacity (FVC), and Mid Expiratory Flow Rate (FEF 25-75). The device can be simple to use and portable and/or hand held.
In a first embodiment, a device is provided that includes:
(a) a conduit having an inlet (or mouthpiece) for exhaled air and an outlet (or flow chamber) for the exhaled air;
(b) a plate (or orifice or closure or sensing) member (or vane) movably disposed in the conduit between the inlet and outlet, the plate member at least partially blocking the conduit and moving in response to the passage of the exhaled air through the conduit; and
(c) a measuring device for measuring, at a plurality of points in time, at least one of (i) a location of the plate member, (ii) a pressure or force applied by the exhaled air against the plate member, and/or (iii) an air flow parameter (e.g., a rate of volume of flow) and generating a plurality of measurement signals. The plate member is typically shaped such that air flow from the inlet end past the member causes the member to move, e.g., rotatably or linearly, away from a rest or starting position to a succession of other open positions forming an ever widening gap between the member and a part or wall of the conduit or passageway. From the position of the plate member or the force applied to the plate member, the pressure applied to the plate (e.g., the back pressure or the pressure or force applied by the air flow) in front of the plate member can be determined.
In one configuration, the device measures the force applied to the plate by the mass of air contacting the plate (even though there is no back pressure). The force is directly proportional to the flow rate or the kinetic energy of the air flow.
In another configuration, the outlet is at ambient (atmospheric) pressure and therefore the pressure differential across the plate member can be determined. The use of the displacement measuring device thereby eliminates the need for an expensive pressure transducer and related electronics. The device can be designed to comply with the stringent maximum back pressure requirements of the American Thoracic Society (which require the back pressure to be less than about 2.5 cm H2O/liter second measured at 14 liters/second airflow (for monitoring applications)) and 1.5 cm H2O/liter second measured at 14 liters/second airflow (for diagnostic applications)), but also, by selecting the distance between the displacement member and the outlet of the conduit, the relationship between the pressure and air flow for the device can be predetermined (e.g., the shape of the curve when pressure is plotted against flow rate can be controlled). In this manner, extremely low flow rates can be accurately measured.
To permit determination of time dependent parameters, such as FEV1, or FEV6, the device can further include a processing unit for receiving the plurality of measurement signals and an electronic memory, in communication with the processing unit, for recording the location of or pressure or force applied to the plate member at the plurality of points in time. This permits determination of the flow rates at the differing points in time and, therefore, the volumetric flows over a selected time interval. The contents of the electronic memory can be read by the user through a visual display and/or uploaded to a computer to generate an electronic diary for the patient and/or to forward the information by modem to a physician. Physicians can program the device to set goals or targets using a computer (PC) interconnected via a port to the device or keys on the device. In one configuration, the memory is tamper proof, thereby eliminating errors that frequently are associated with manually logged results.
The measuring device can be any suitable device for monitoring the pressure or force applied to the plate member and/or the plate member position as a function of time, such as a strain gauge (e.g., a single, half or full resistor bridge strain gauge), a radiant energy source (e.g., a light or sound energy source) in communication with a radiant energy detector. In one configuration, the device is any type of strain gauge, such as piezoresistive, thin film, semiconductor, and the like, that measures deformation of a plate. A particularly preferred strain gauge has an active circuit and an inactive circuit. By configuring the strain gauge as a half or full resistor bridge, noise as a result of thermal expansion or contraction of the plate member and the like, can be zeroed out. The strain gauge is typically located on an upstream (front) or downstream (rear) surface of the plate member relative to the direction of exhaled air flow.
In another embodiment, a method is provided for determining exhaled air flow. The method includes the steps of:
(a) exhaling air into an inlet of a conduit;
(b) moving a sensing member that is movably disposed in the conduit downstream of the inlet, the sensing member at least partially blocking the conduit and moving in response to the passage of the exhaled air through the conduit; and
(c) measuring the location of the sensing member at a plurality of points in time and generating a plurality of location signals. The plurality of location signals can be processed to determine a desired air flow parameter.
In another embodiment, a portable device for measuring respiratory air flow is provided that compensates for the effects of inertia of the plate member. The device includes a self-oscillation dampener (or dampening means) that resists movement of the sensing member. In one configuration, the dampener is located behind and movably (e.g., frictionally) engages the sensing member. The self-oscillation dampener dampens the amplitude of oscillations of the sensing member in response to exhaled air contacting the sensing member. The self-oscillation dampener can also increase the resonant frequency of the sensing member such that the resonant frequency exceeds the frequency of any oscillations imparted to the system by the air flow. In one configuration, the self-oscillation dampener is located on the downstream side of the sensing member. In another configuration, the self-oscillation dampener applies a pressure to the sensing member of no more than about 10 gm or no more than about 10% of the pressure applied to the sensing member by the air flow.
In yet another embodiment, a device for measuring respiratory air flow is provided that includes:
(a) a conduit having an inlet for exhaled air and an outlet for the exhaled air;
(b) a plate member movably disposed in the conduit between the inlet and outlet, the plate member at least partially blocking the conduit and moving in response to the passage of the exhaled air through the conduit; and
(c) a measuring device for measuring at least one of a pressure or force applied against the plate member by the exhaled air and generating a measurement signal. The measuring device is located on (e.g., adhered or attached to, etched on, etc.) or otherwise engages the plate member. In a particularly preferred configuration, the measuring device is a strain gauge.
In a further embodiment, a portable device is provided that increases the plate member""s resonant frequency for the reasons noted above. The device includes (incorporates) or engages one or more stiffening members to impart rigidity to the plate member without significantly increasing the mass of the member. The stiffening members can be located anywhere on the plate member such as on a peripheral edge(s) of the plate member and/or in the central portion of the plate member. By increasing the rigidity of the plate member, the stiffening members also inhibit or minimize flutter of the member in response to air flow.
In another embodiment, a portable device for measuring respiratory air flow is provided that substantially eliminates the effect of gravity on the air flow measurement. The device includes
(a) a conduit having an inlet for exhaled air and an outlet for the exhaled air; and
(b) a sensing member for measuring an air flow parameter (e.g., a pressure transducer, a movable plate). The direction of air flow through the inlet is transverse to the direction of air flow at the sensing member. In one configuration, the direction of air flow through the inlet is substantially normal to the direction of air flow at the sensing member. In this design, the positioning of the device when the patient exhales into the conduit has substantially no effect on the air flow measurement. In one configuration, an axis of sensing member movement is substantially normal or orthogonal to an axis of possible movement of the patient during the respiratory test using the device.
In yet a further embodiment, the device includes a detachable or removable head assembly that includes the input conduit, sensing member and outlet conduit. The head assembly is removable for cleaning. In this manner, the device does not require a bacterial filter.
The above-described embodiments and configurations are neither complete nor exhaustive. As will be appreciated, other embodiments of the invention are possible utilizing, alone or in combination, one or more of the features set forth above or described in detail below.